The present invention generally relates to semiconductor devices, and more particularly to a semiconductor device utilizing multiquantum wells.
Generally, a quantum well refers to a square potential well having a potential of such a size that electrons or holes confined therein show a quantum effect, and quantum states of different energies (levels) are generated within the quantum well. For example, when two quantum wells are formed adjacent to each other, carriers may with a certain rate (probability) move by tunneling between the quantum wells. In this case, the tunneling rate of the carriers at a certain quantum level in a first quantum well becomes large when a second quantum well has a quantum level identical to or close to that of the first quantum well, and becomes small when the second quantum well does not have a quantum level close to that of the first quantum well. Theoretically, when a plurality of such quantum wells are arranged and a bias voltage is applied across two ends of the quantum well arrangement, it is possible to generate a tunnelling simultaneously in all of the quantum wells at a certain bias voltage. By using this phenomenon, it is possible to obtain elements such as a negative resistance element, a semiconductor laser having a high quantum efficiency and capable of operating at a high speed, and a photodiode having a high sensitivity and capable of operating at a high speed.
The conventional semiconductor element such as the negative resistance element either uses no quantum well or simply uses a single quantum well. For this reason, the negative resistance element suffers the following deficiencies.
For example, in the case of an Esaki diode having a p.sup.+ -n.sup.+ junction, which is a kind of negative resistance element, it is necessary to heavily dope the p.sup.+ -type and n.sup.+ -type regions of the Esaki diode. However, it is difficult to control such a heavy doping, and the depth of the p.sup.+ -n.sup.+ junction cannot be accurately controlled. In addition, to avoid a change in the current versus voltage characteristic, the Esaki diode should be operated below a certain temperature such that the effects of impurity diffusion and drift can be neglected.
Similarly, as another kind of negative resistance element, there is a resonant tunnel diode which uses a single quantum well, that is, double potential barriers, but a ratio between a peak current at a resonant point and a valley current at a non-resonant point is small, because the valley current is added with a current component of inelastic tunnel. The inelastic tunnel refers to the tunnel caused by the inelastic scattering of the carriers.
In addition, in the normal semiconductor laser, there are problems in that the wavelength of the emitted light is limited by the energy band gap, the light emitting operation is slow because it is dependent on the p-n recombination rate, the half-width and monochromaticity are poor because a transition takes place among a plurality of different quantum states, and the frequency of the light modulation is governed by the carrier storage time and cannot be increased.
Furthermore, the photodiode does not operate unless the bias voltage is set to a sufficiently large voltage, and it is difficult to operate the photodiode at a high speed since the operation speed is determined by the transit time of the carriers in a depletion region.
Many of the problems described heretofore can be eliminated theoretically by providing multiquantum wells. But in actual practice, it is difficult to make the carriers tunnel through the multiquantum wells with a high rate, because it becomes difficult to match the quantum levels in the multiquantum wells as the number of quantum wells becomes large due to the fact that the multiquantum wells are square wells.